Iran’s unit of currency is the rial. The official exchange rate averaged 9,171 rials to the U.S. dollar in 2006. However, rials are exchanged on the unofficial market at a much higher rate. In 1979 the government nationalized all private banks and announced the establishment of a banking system whereby, in accordance with Islamic law, interest on loans was replaced with handling fees; the system went into effect in the mid-1980s. The banking system consists of the central bank, which issues currency; several commercial banks that are headquartered in Tehrān but have branches throughout the country; two development banks; and a housing bank that specializes in home mortgages. The government began to privatize the banking sector in 2001, when it issued licenses to two new privately owned banks. The Tehrān Stock Exchange trades the shares of more than 400 registered companies.